A grow-box-spacer-pane% object is intended for use as a
lightweight spacer in the bottom-right corner of a frame, rather than
as a container. On older version of Mac OS X, a
grow-box-spacer-pane% has the same width and height as the
grow box that is inset into the bottom-right corner of a frame. On
Windows, Unix, and recent Mac OS X, a grow-box-spacer-pane% has zero width and
height. Unlike all other container types, a
grow-box-spacer-pane% is unstretchable by default.